1. Field of the Invention
This invention relates to the field of electrostatography, and more particularly, to improvements in a method for controlling toner replenishment.
2. Description of the Prior Art
Toning stations for electrophotographic copiers and printers typically have two-component developer mixtures (carrier and toner). Toner depleted by toning latent images on the photoconductor must be replaced by replenishing with new toner, so that the toner concentration (TC) remains within a usable range in the toning station developer mix.
Closed-loop toner concentration control, for example, see U.S. Pat. No. 4,875,078, is typically achieved by means of a TC monitor, and control logic to drive a toner replenishment mechanism. TC monitors are of several types, including optical and magnetic. A limitation on the performance of such TC monitors is that their sensitivity varies greatly from unit to unit and over age. Mounting variability of the sensor probe on the development station as well as variability in the sensor probe itself contributes to overall variability. Another limitation is in the replenisher mechanism, where again there is substantial variability in toner delivery rate (gain), unit to unit and over time. Current practice is to adjust the monitor output, V.sub.MON, to 2.500 V when a new load of developer at nominal 10% concentration is installed in the development station. The replenishment algorithm then acts to regulate V.sub.MON to this initial 2.500 V value. Maintaining V.sub.MON =2.500 V assures that TC=10% (barring monitor drift) regardless of TC monitor sensitivity.
With reference now to FIG. 7, there is shown a schematic of an electrophotographic copier/printer apparatus of the prior art having one form of control system for replenishing toner taken out during the reproduction cycle. The apparatus 10 comprises a moving belt 18 entrained about rollers 11-17, one of which is driven by a motor M to drive the belt in the direction indicated by the arrow. A corona charger 19 provides a uniform electrostatic charge on the belt. An electro-optical exposure source 20 exposes the belt to form an electrostatic image that is developed with toner particles from a station 22. The developed image is then transferred to a sheet S at a transfer station 24 and the toner image is fused to the sheet by fusing rollers 26. In order to control the concentration of toner particles in the developer mix (magnetic carrier particles plus non-magnetic toner particles) a toner concentration monitor 30 is provided having a probe 30a mounted either inside or outside of the development station's housing 22. In response to toner concentration as sensed by the probe, a signal V.sub.MON is generated by the TC monitor 30. In the prior art, the TC monitor outputs a signal V.sub.MON in accordance with an assumed or nominal sensitivity (S.sub.NOMINAL) having the parametric relationship of V.sub.MON to TC illustrated in the accompanying FIG. 8. The signal V.sub.MON output by the TC monitor 30 is then compared with the set point of 2.5 volts and an error signal E is generated that may be input to a replenishment motor control unit 32 which controls the duty cycle of a replenishment motor 34. The motor 34 drives a toner auger 36 that feeds replenishment toner into the development station 22. The toner is mixed with the carrier particles in the development station by suitable mixing blades as is well known to obtain a uniform mixture. As noted in U.S. Pat. No. 4,875,078, improvements in control of toner concentration may be provided by providing a proportional plus integral control of the error signal E. In any event, a closed loop control of toner concentration is provided. In order to guard against harmful extremes of TC, upper and lower limits are set for V.sub.MON (see V.sub.MON1 and V.sub.MON2). The corresponding values for TC, however, depend on the actual sensitivity and not those derived using the nominal sensitivity parametric relationship illustrated in FIG. 2. So the limits on V.sub.MON are set to accommodate the worst case, and are reached prematurely for the nominal toner monitor sensitivity. Another problem arises when it is desirable to change TC from the nominal 10% at setup. This may be done by changing the aim voltage in the replenisher algorithm from the initial 2.500 to a new value. The actual TC corresponding to the new V.sub.MON aim value depends on the actual parametric relationship between TC and V.sub.MON or actual sensitivity. Given the wide distribution of possible actual sensitivity values, there is a substantial likelihood of error in forming the new intended TC aim point.
The logic of the TC control algorithm is typically designed for a nominal monitor sensitivity and replenisher gain. Departure from these nominal values degrades the accuracy of the TC regulation. Alternatively, the algorithm may be designed to assure acceptable performance with extreme values of monitor sensitivity and/or replenisher gain, at some sacrifice of performance with nominal values.